In fuel cells as they are commonly used in motor vehicles, in particular in the case of fuel cells having a proton-conducting polymer membrane, which are also known under the English term of proton exchange membrane fuel cell (PEM-FC) or polymer electrolyte fuel cell, the membrane forming the electrolyte must be kept moist at all times during operation. If the moisture drops below a certain value, the ion conductivity of the membrane decreases. To keep the moisture of the fuel cell membrane at a specific optimum level, deionized water is often metered to the supplied gas flow.
The German Published Patent Application No. 199 53 803 describes a device for moistening the gas flow where water is added to the gas flow via a simple nozzle that projects into the gas flow.
However, the water should be metered into the gas flow as precisely as possible as a function of the gas flow across a broad dosing range, largely independently of the water-pump pressure, and it should be able to be regulated across a plurality of parameters in a cost-effective and reliable manner. For these reasons, the use of fuel injectors, for instance, which are already known from reciprocating engines having internal combustion, is advantageous for the metering of water. Such a valve is known from German Published Patent Application No. 199 53 803, for example.
A similar type of application is the precise metering of a watery urea-water solution to reduce the nitrogen oxides in the exhaust tract of diesel vehicles for exhaust-gas aftertreatment or in the case of generally non-lubricatable media.
For the system known from German Published Patent Application No. 199 53 803, disadvantages result that are mainly due to the fact that the device disclosed there has been optimized for the processing of fuels, which have considerably different chemical properties than water. For instance, most fuels such as gasoline have their own lubrication characteristics and have an inhibiting effect on chemical corrosion or do not promote chemical corrosion by themselves, in particular on metallic surfaces. Water, on the other hand, has no intrinsic lubricating characteristics and promotes chemical corrosion on metallic surfaces, in particular on metal surfaces containing iron. In the fuel injector mentioned, such iron-containing metallic surfaces are quite frequently in contact with the particular fluid to be injected.
It is also disadvantageous that the known fuel injector is designed for use in higher temperature ranges such as above 100° C. For that reason, metallic materials, which have excellent thermal resistance, were used in the valve-sealing seat in the manufacture of the fuel injector. However, the use of thermally resistant material such as iron-containing metal in the region of the sealing seat allows only a certain measure of tightness of the sealing seat, even when cost-intensive small manufacturing tolerances are used. Furthermore, due to their lack of elasticity, thermally resistant metals increase the forces acting in the sealing seat or in the force-transmitting components during the valve opening and closing operations.